The present invention refers to a construction for a brushless electric motor rotor comprising a metallic core, which carries, on its lateral surface, circumferentially disposed magnets.
In the known constructions for a brushless electric motor rotor, the permanent magnets, usually in the form of arcuated plates, are retained on a cylindrical core, usually made of iron, which may be laminated or massive and which is mounted around the motor shaft.
In these rotors, the magnets are provided in housings defined by longitudinal openings inside the metallic core or kept seated on the cylindrical lateral surface of the core through different fixation means, which are designed to impart to the mechanical structure the necessary resistance to centrifugal forces and to the motor operation.
In the rotor construction in which the magnets are affixed to the cylindrical lateral surface, one of the assembly problems results from the need to maintain the longitudinal axis of the magnets parallel to the longitudinal axis of the core. This positioning is usually obtained by providing external radial saliences incorporated to the surface of the rotor core or by equipments for carrying out the assembly of the unit during the manufacturing process.
These techniques have the inconvenience of making difficult the positioning of the magnets on the rotor surface during manufacture, which positioning should be angularly correct and provide a determined angular distance between the magnets. This difficulty is due to the degree of longitudinal freedom existing during assembly.
The positioning of the magnets on the rotor surface by means of external radial saliences of the core causes an assembly difficulty, which resides on the fact that the magnets are supported on only one of said saliences, since, during the operation of the motor or also during the mounting process of said rotor, the different thermal expansion of both the magnets and the core may generate high mechanical stresses concentrated on the magnets at the supporting region, in case the magnets are simultaneously supported on two opposite saliences. These stresses may cause failures caused by magnet breakage, as they are formed of ceramic material. In certain cases, even the support on only one salience can be critical.
After the formation of the rotor, with the magnets correctly positioned around the core, the rotor has to be positioned and mounted around the motor shaft, when positioning means are required to align the rotor during its assembly phase around the motor shaft. These positioning means also position the rotor for the posterior magnetization of the magnets, which has to be effected in a predetermined position of said magnets in relation to the motor structure. This positioning function is usually provided by bores, which are made in the magnetic core of the rotor and which reduce the amount of active material of said core, impairing the efficiecy of the motors or limiting the minimum size that the rotors may have, without the bores impairing relevantly the performance of said motor.
Another deficiency of the prior art is the high manufacturing cost of the rotors, as a function of the high investments required, when usually automatic positioning equipments are used.
Thus, it is an objective of the present invention to provide an electric motor rotor with permanent magnets, which allows the magnets to be precisely seated onto the external surface of the rotor core, without requiring constructive changes in the core or precision equipments.
Another objective of the present invention is to provide an electric motor rotor of the type having permanent magnets and which allows to simplify the automatic mounting process of the rotor.
These and other objectives are attained by an electric motor rotor with permanent magnets, said rotor including a core having a cylindrical lateral surface, against which are seated magnets having opposite lateral edges, said rotor comprising a pair of annular caps, each cap being seated and affixed onto an adjacent end face of the core, said annular caps limiting the axial displacements of the magnets and defining, for both directions of circumferential displacement, stops for this displacement for each magnet, the confronting lateral edges of a pair of consecutive magnets being positioned by said annular caps, in order to define a previously established minimum circumferential distance.